A key step in the development of inner ear sensory epithelia is the commitment of multipotent progenitor cells in otocyst to a prosensory fate, defined by the ability to give rise to either hair cells or support cells. However, the factors that confer prosensory fate to these cells are only beginning to be identified. Recent studies have shown that the HMG class transcription factor Sox2 is dynamically expressed in prosensory cells and is required for the formation of this cell type. However, the specific roles of Sox2, and in particular whether it can induce prosensory identity, has not been determined. The main goals of the experiments described in this proposal are to use a combination of in vivo and in vitro approaches to determine the specific effects of Sox2 during cochlear development and further, to determine whether some or all of these effects are mediated through the candidate downstream effectors, Proxl, E-cadherin, and N-cadherin. During the K99 phase of the award, progress has been made towards the completion of all three aims. The first aim that was proposed as part of K99 focussed on the specific roles of Sox2 in regulating prosensory, and ultimately hair cell specification. Our results demonstrate that transient activation of Sox2 leads to ectopic hair cell formation, suggesting a direct role of Sox2 in the induction of Atohl, a conclusion that was confirmed by chromatin immunoprecipitation analysis. In addition, our data show that induction of prosensory identity is essential for hair cell formation and that hair cells must go through a prosensory cell phase prior to hair cell differentiation. In the second aim, the relationship between Sox2 and the putative Sox2-target gene, E-cadherin, will be examined. Gain- & loss-of function in vivo and in vitro experiments will be used to determine whether the effects of Sox2 on cell fate and patterning are mediated through E-cadherin. In the final aim, the role of N-cadherin, a known target of Sox2, in determination of cell fate and cellular patterning will be examined. Moreover, experiments examining the specific effects of modulation of E-cadherin and Ncadherin signaling & the relationship of these two molecules to Sox2 should provide insights into the signaling pathways that direct cells towards hair cell fate.